1. Field of the Invention
The present invention relates to novel magnet-superconductor systems. In another aspect the present invention relates to magnet-superconductor systems useful for controlling and/or influencing relative motion between two or more members. In still another aspect the present invention relates to vibrational damping systems, brakes, clutches, and other articles which utilize magnet-superconductor systems.
2. Description of the Related Art
Many types of prior art devices exist for controlling or influencing relative motion between two or more bodies. For example, vibration damping systems, brakes and clutches are well known and generally utilize mechanical type of apparatus that are subject to wear, noise, vibration and friction heating problems. These problems can often lead to seizure or other failure of the apparatus. For example, springs or shock absorbers utilized in conventional vibrational control systems tend to wear out with repeated usage over time. Brake pads and mechanical clutches ultimately wear out, and have to be periodically replaced. In additional, mechanical systems often require lubricants which fail in severe environments such as those commonly encountered in outer space. Failure of conventional liquid lubricants in outer space is usually due to the vacuum conditions that cause the lubricants to outgas or vaporize, leaving contact surfaces dry and resulting in the ultimate failure of the apparatus. Additionally, in outer space temperatures are very low so most lubricants solidify and simply do not function as lubricants.
As an alternative to mechanical systems for controlling or influencing motion, magnetic systems have been developed. Conventional magnetic systems for controlling or influencing motion are generally unstable and require for their operability control means, such as rapidly acting feedback control systems, to compensate for displacements from the set point. Until recently, such magnetic systems have utilized magnets of two types--either permanent magnets or electromagnets.
Because field strength of permanent magnets is generally limited, the use of permanents magnets is limited to applications where very small forces are adequate. Electromagnets, which can supply considerably more magnetic force than comparable permanent magnets, are much more convenient to use and are thus preferred for use in conjunction with feedback control systems. However, because of the required feedback control systems, use of electromagnets adds considerably to the cost, size, and operational complexity of the system.
It has been appreciated for years that magnetic fields strongly interact with superconducting materials. The most distinctive property of a superconductive material is its loss of electrical resistance when it is at or below a critical temperature. This critical temperature is an intrinsic property of the material and is referred to as the superconducting transition temperature of the material, T.sub.c.
There have been recent advances in superconducting materials and parallel advancements in the field of permanent magnets. Superconductive materials are of two basic types, designated as Type I and Type II. Unlike Type II superconductors, Type I superconductors are incapable of effecting stable suspension.
Type I superconductors feature perfect diamagnetism up to a critical applied field, at which point superconductivity is lost and the magnetization of the sample abruptly disappears. Examples of applications of Type I materials can be found in U.S. Pat. Nos. 3,493,274 and 3,026,151, which disclose a bearing utilizing Type I materials. In order to achieve stability in these systems, the bearing structures must rely on either a mechanical rotary support or must employ superconductors shaped to provide a laterally stable configuration.
The recent discoveries of high temperature superconductors involve Type II materials. Whereas a Type I superconductor completely blocks out magnetic flux from its interior, a phenomenon known as diamagnetism, Type II superconductors allow a certain amount of magnetic flux to penetrate into the interior of the material, producing a stable suspension effect in addition to a levitation effect. Under such conditions, circulating superconducting currents are established within the superconductor.
Recent research activities have brought the discovery of "high temperature superconducting" (HTS) compounds. HTS compounds are those which superconduct at and below a critical temperature, T.sub.c, which is above the boiling point temperature of nitrogen.
Following the discovery of superconductivity in a rare earth-alkaline earth-Cu oxide system of a perovskite crystalline structure, a new class of rare earth-alkaline earth-copper oxides was discovered which are superconductive at temperatures above the boiling point of liquid nitrogen, 77.degree. K. These new rare earth-alkaline earth-copper oxides are now commonly referred to as "123" high-temperature superconductors in reference to the stoichiometry in which the rare earth, alkaline earth, and copper metal atoms are present, namely a ratio of 1:2:3.
Since they are superconductive at temperatures greater than 77.degree. K., the new CuO high temperature superconductors may be cooled with liquid nitrogen, which is a far less costly refrigerant than liquid helium. As a result, the rather complex thermal insulation and helium-recycling systems, necessary to avoid wasting the expensive helium coolant required for the low temperature superconducting material previously known, are no longer necessary. The HTS compounds simplify and enhance the reliability of commercial applications of superconductors. Liquid nitrogen is about 2000 times more efficient to use in terms of cost, when both the refrigerant itself and the associated refrigerant unit design are considered.
A typical example of a system featuring a combination of Type II superconductors and permanent magnets is disclosed in U.S. Pat. No. 4,886,778, which discloses a rotating shaft having two ends, each of which contains a permanent magnet and rotates in a socket clad with superconducting material. The shaft is made to levitate above the sockets by the repulsive forces which exist between the magnets and the superconductors. The incorporation of superconductors into the bearing design offers the possibility of rendering the bearings entirely passive. The design disclosed in U.S. Pat. No. 4,886,778 has the potential for achieving very high rotational speeds, in excess of ten thousand rpm. The interaction between the rotating magnetic axial element and its stationary superconducting support takes place across a gap permeated by a strong magnetic field emanating from permanent magnets embedded in the rotating element.
However, while systems featuring Type II superconductors and magnets have been proposed for systems designed to perpetuate motion, such as bearings, these is a need for superconductor/magnet systems for influencing and/or controlling motion.